In recent years, light sources such as light emitting diodes (LEDs) have been improved through technological advances in material and manufacturing processes. LED possesses relatively high efficiency, long life, vivid colors and can be used in a variety of industries including the automotive, computer, telecom, military and consumer goods, etc. One exemplary application is an LED lamp which uses LEDs to replace traditional light sources such as electrical filament.
FIG. 1 shows a schematic diagram of a conventional LED driving circuit 100. The LED driving circuit 100 utilizes an LED string 106 as a light source. The LED string 106 comprises a group of LEDs connected in series. A power converter 102 converts an input voltage Vin to a desired output DC voltage Vout for powering the LED string 106. A switch 104 coupled to the power converter 102 can enable or disable the input voltage Vin to the LED string 106, and therefore can turn on or turn off the LED lamp. The power converter 102 receives a feedback signal from a current sensing resistor Rsen and adjusts the output voltage Vout to make the LED string 106 generate a desired light output. One of the drawbacks of this solution is that a desired light output is predetermined. In operation, the light output of the LED string 106 is set to a predetermined level and may not be adjusted by users.
FIG. 2 illustrates a schematic diagram of another conventional LED driving circuit 200. A power converter 102 converts an input voltage Vin to a desired output DC voltage Vout for powering the LED string 106. A switch 104 coupled to the power converter 102 can enable or disable the input voltage Vin to the LED string 106, and therefore can turn on or turn off the LED lamp. The LED string 106 is coupled to a linear LED current regulator 208. Operational amplifiers 210 in the linear LED current regulator 208 compares a reference signal REF and a current monitoring signal from current sensing resistor Rsen, and generates a control signal to adjust the resistance of transistor Q1 in a linear mode. Therefore, the LED current flowing through the LED string 106 can be adjusted accordingly. In this solution, in order to control the light output of the LED string 106, users may need to use a dedicated apparatus, such as a specially designed switch with adjusting buttons or a switch that can receive a remote control signal, to adjust the reference signal REF.